<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        *{
            margin: 0;
            padding: 0;
        }
        html,body{
            height: 100%;
        }
        bady{
            display: flex;
            flex-direction: column;
        }
        header{
            height: 45px;
            background-color: gray;
        }
        section{
         flex:1;
        }
        footer{
            height: 45px;
            background-color: rgb(161, 161, 161);
        }
    </style>
</head>
<body>
    <header></header>
    <section></section>
    <footer></footer>
</body>
</html>